1. Field of the Invention
This invention relates to a squeezing detection control method for consumable electrode arc welding, to detect squeezing phenomena of droplets during short circuiting periods in consumable electrode arc welding, in order to sharply reduce the welding current and improve welding quality.
2. Description of the Related Art
FIG. 5 is a diagram showing current and voltage waveforms and droplet transfer in consumable electrode arc welding in which short circuiting periods Ts and arc periods Ta are repeated. In the figure, (A) indicates the change with time in the welding current Iw passing through the consumable electrode (hereafter called the welding wire 1), (B) indicates the change with time in the welding voltage Vw applied across the welding wire 1 and base material 2, and (C) through (E) indicate the manner of transfer of droplets 1a. 
During the short circuit period Ts between times t1 and t3, a droplet 1a at the tip of the welding wire 1 is in a short circuiting state with the base material 2, and as shown in (A) in the figure, the welding current Iw gradually increases, so that as shown in (B) in the figure the welding voltage Vw assumes a low value of approximately several volts due to the short-circuited state. As shown in (C) in the figure, at time t1 the droplet 1a comes into contact with the base material 2 and enters a short circuiting state. Thereafter, as shown in (D) in the figure, squeezing 1b occurs in the upper portion of the droplet 1a due to the electromagnetic pinching force resulting from the welding current Iw passing through the droplet 1a. This squeezing 1b progresses rapidly, and as shown in (E) in the figure, at time t3 the droplet 1a is transferred from the welding wire 1 to the molten pool 2a, and the re-striking of an arc 3 occurs.
When the above squeezing phenomenon occurs, the short circuit is opened after an extremely short time on the order of several hundred μs, and an arc 3 re-strikes. That is, this squeezing phenomenon is a precursor of the opening of the short circuit. When squeezing 1b occurs, the conduction path of the welding current Iw becomes narrow at the squeezed portion, so that the resistance of the squeezed portion increases. The resistance increases as the squeezed portion becomes narrower with the progress of the squeezing. Hence by detecting a change in the resistance between the welding wire 1 and base material 2 during the short circuit period Ts, the occurrence and progress of the squeezing phenomenon can be detected. The change in resistance can be calculated by computing (welding voltage Vw)/(welding current Iw). Further, as explained above, the squeezing occurrence duration is an extremely short length of time, so that as shown in (A) in the figure, the change in welding current Iw during this period is small. Hence in place of a change in the resistance, occurrence of the squeezing phenomenon can also be detected through a change in the welding voltage Vw. One specific method of squeezing detection involves computing the rate of change (differential value) of the resistance or the welding voltage Vw during a short circuit period Ts, and detecting squeezing when this rate of change has reached a squeezing detection reference value determined in advance. Another method involves computing the voltage increase ΔV from the stable short circuit voltage Vs prior to the occurrence of squeezing during a short circuit period Ts, as in (B) in the figure, and detecting squeezing when at time t2 this voltage increase ΔV has reached a squeezing detection reference value Vtn determined in advance. In the following explanation, a case in which the above squeezing detection method employing the voltage increase ΔV is assumed; but various other methods proposed in the prior art may be used. The re-striking of an arc at time t3 can easily be detected by judging that the welding voltage Vw has become equal to or greater than a short circuit/arc discrimination value Vta. The period in which Vw<Vta is the short circuit period Ts, and the period in which Vw≧Vta is the arc period Ta. The time from detection of the occurrence of squeezing between times t2 and t3 and the time of re-striking of an arc will hereafter be called the squeezing detection period Tn. When at time t3 an arc re-strikes, the welding current Iw rises rapidly and then gently declines, as shown in (A) in the figure; and as indicated in (B), the welding voltage Vw becomes an arcing voltage of magnitude approximately several tens of volts. During the arc period Ta between times t3 and t4, the tip of the welding wire 1 is molten and a droplet 1a forms. Thereafter, the operation from time t1 to time t4 is repeated.
In the above-described welding accompanied by short circuiting, the current at time t3 when the arc 3 re-strikes can be a large current. In that (case, the arc force from the arc 3 toward the molten pool 2a rapidly increases, and massive sputtering occurs. That is, the amount of sputtering increases substantially in proportion to the current at arc re-striking Ia. Hence in order to suppress the occurrence of sputtering, this current at arc re-striking Ia must be kept small. As methods to accomplish this, various welding power supplies have been proposed in the prior art in which occurrence of the squeezing phenomenon is detected, and the welding current Iw is rapidly decreased to reduce the current at arc re-striking Ia. Below, this technology of the prior art is explained.
FIG. 6 is a block diagram of a welding power supply which adopts a squeezing detection control method of the prior art. The welding power supply PS is a welding power supply for use in general consumable electrode arc welding. The transistor TR is inserted in series with the output, and the resistor R is connected in parallel therewith. The voltage detection circuit VD detects the welding voltage Vw and outputs a voltage detection signal Vd. The squeezing detection circuit ND takes this voltage detection signal Vd as input, and outputs a squeezing detection signal Nd which is set to high level when in the short circuiting period Ts the above-described voltage increase ΔV reaches a squeezing detection reference value Vtn determined in advance, and which is set to low level when the voltage detection signal Vd reaches a short circuit/arcing discrimination value Vta determined in advance. That is, this squeezing detection signal Nd is at high level during the above-described squeezing detection period Tn. The driving circuit DR outputs a driving signal Dr which turns on the transistor TR when this squeezing detection signal Nd is at low level (when squeezing is not detected). Hence the transistor TR is in the off state when the squeezing detection signal Nd is at high level (when squeezing is detected).
FIG. 7 is a timing chart of the various signals in the above welding power supply. In the figure, (A) shows the welding current Iw, (B) shows the welding voltage Vw, (C) shows the squeezing detection signal Nd, and (D) shows the driving signal Dr.
In the figure, at the periods other than the squeezing detection period Tn from times t2 to t3, the squeezing detection signal Nd is at low level, as shown in (C); hence as indicated in (D), the driving signal Dr is at high level. As a result, the transistor TR is in the on state, so that operation is the same as that of a welding power supply for normal consumable electrode arc welding.
At time t2, when as shown in (B) in the figure the welding voltage Vw rises in the short circuiting period Ts and the volt-age increase ΔV is detected as having become equal to or greater than a squeezing detection reference value Vtn determined in advance, so that droplet squeezing is judged to have occurred, the squeezing detection signal Nd goes to high level, as in (C) in the figure. In response to this, as shown in (D) in the figure, the driving signal Dr goes to low level, and so the transistor TR enters the off state. As a result, the resistor R is inserted into the conduction path of the welding current Iw. The value of this resistor R is set to a value ten times or more than the short circuit load (several tens of mΩ), so that as shown in (A) in the figure, the energy stored in the DC reactor within the welding power supply and the cable reactor is suddenly discharged, and the welding current Iw decreases rapidly. At time t3, when the short circuit is opened and arcing again occurs, the welding voltage Vw becomes equal to or greater than the short circuiting/arcing discrimination value Vta, determined in advance, as shown in (B). Upon detection of this, the squeezing detection signal Nd goes to low level, as shown in (C), and the driving signal Dr goes to high level, as shown in (D). As a result, the transistor TR enters the on state, and normal consumable electrode arc welding control is performed. By means of this operation, the arc re-striking current Ia at the time an arc re-strikes (at time t3) can be made small, and the occurrence of sputtering can be suppressed.
The above explanation is for the case of DC consumable electrode arc welding; but the case of consumable electrode arc welding accompanying short circuiting is similar. Below, a squeezing detection control method for consumable electrode arc welding is explained.
FIG. 8 is a current and voltage waveform diagram showing a squeezing detection, control method for consumable electrode arc welding. In the figure, (A) is a polarity-switching signal Spn, (B) is the welding current Iw, and (C) is the welding voltage Vw.
As shown in (A) in the figure, the polarity-switching signal Spn is at high level during an electrode positive polarity period Tep, determined in advance, and is at low level during an electrode negative polarity period Ten, determined in advance. The output polarity of the welding power supply is switched according to this polarity-switching signal Spn. In (B) and (C) in the figure, 0 A or 0 V and above indicate electrode positive polarity EP, and values below these indicate electrode negative polarity EN. Further, unless stipulated otherwise, the values of the welding current Iw and welding voltage Vw represent absolute values for both of the polarities.
When short circuiting occurs at time t1 during an electrode positive polarity period Tep, the welding current Iw increases, as shown in (B) in the figure, and as shown in (C), the welding voltage Vw becomes a low short circuit voltage value Vs of approximately several volts. When squeezing occurs at the droplet during the short circuit period Ts, the welding voltage Vw increases as shown in (C), and at time t2 the voltage increase ΔV reaches the squeezing detection reference value Vtn. In response, as shown in (B) in the figure, the welding current Iw falls rapidly. Then, at time t3 an arc re-strikes. The current Ia at the time of arc re-striking is low, so that there is extremely little occurrence of sputtering. During the arc period Ta, as shown in (B) in the figure, the welding current Iw rises rapidly and then falls gently, and as shown in (C), the welding voltage Vw assumes an arcing voltage value of several tens of volts. During the electrode positive polarity period Tep, the above operation is repeated. The electrode positive polarity period Tep is often set to approximately several hundred milliseconds, and so the number of short circuits during one period is approximately from several times to several tens of times.
At time t5, as shown in (A) in the figure, the polarity-switching signal Spn changes to low level, and the welding power supply output polarity switches to electrode negative polarity EN. At time t5, short circuiting occurs, and upon entering the short circuiting period Ts, the welding current Iw increases and the welding voltage Vw becomes the low short circuit voltage Vs, similarly to the above operation. Droplet squeezing occurs, and at time t6, when the voltage increase ΔV reaches the squeezing detection reference value Vtn as in (C), the welding current Iw drops rapidly, as shown in (B). Then, when arc re-strikes at time t7, the welding current Iw rises rapidly and then falls gradually as shown in (B), and as shown in (C), the welding voltage Vw assumes an arcing voltage value of several tens of volts. In this case also, the arc re-striking current Ia at time t7 is low, so that there is extremely little occurrence of sputtering. During the electrode negative polarity period Ten, the above operation is repeated. This electrode negative polarity period Ten is also set to approximately several hundred milliseconds, so that the number of short circuits in one period is, similarly to the above, from several times to several tens of times approximately.
As explained above, by performing squeezing detection control in consumable electrode arc welding also, the occurrence of sputtering can be greatly reduced, and high-quality welding becomes possible.
In the above-described squeezing detection control, the accurate detection of occurrence of the squeezing phenomenon is essential to greatly reduce sputtering and enable high-quality welding. Hence the squeezing detection sensitivity (the setting for the squeezing detection reference value Vtn) must be made appropriate for various welding conditions. Welding conditions include the material of the object for welding, joints, welding attitude, wire protrusion length, feed rate, welding rate, and numerous other parameters. In order to set the squeezing detection reference value Vtn appropriately for each of these welding conditions, in the prior art, a method is used in which the squeezing detection period Tn or current at arc re-striking Ia is used in feedback control to automatically adjust the squeezing detection reference value Vtn so as to attain a target value, as shown in FIG. 7. Further, in some cases a knob for adjusting the squeezing detection reference value Vtn is provided on the panel of the welding power supply. (As examples of the prior art see Japanese Patent Laid-open No. 2004-114088 and Japanese Patent Laid-open No. 2006-281219.)
In the above-described squeezing detection control method for consumable electrode arc welding of the prior art, the absolute value of the AC welding voltage Vw is detected and is used in constant-voltage control and squeezing detection control in the welding power supply. This is because using the DC signal facilitates the processing in the control circuit. Hence in a method of squeezing detection control for consumable electrode arc welding also, normally the squeezing detection reference value Vtn is set to one value for each set of welding conditions. For this reason, the squeezing detection reference value Vtn has been set to the same value during both electrode positive polarity periods Tep and during electrode negative polarity periods Ten.
However, the state of droplet formation and state of squeezing occurrence differ greatly for electrode positive polarity EP and for electrode negative polarity EN. As a result, if the squeezing detection reference value Vtn, which is the squeezing detection sensitivity, is set appropriately for electrode positive polarity EP, then a state ensues which is inappropriate for electrode negative polarity EN, and similarly for the reverse case. Moreover, even when the above-described method for appropriate selection of a squeezing detection reference value Vtn is used, the value is made appropriate to each set of welding conditions, but is not made appropriate to each polarity. For this reason, in consumable electrode arc welding, there are cases in which the effect in reducing the occurrence of sputtering is insufficient.